This file photo taken on October 8, 2016 shows House Speaker Paul Ryan speaking during the 1st Congressional District Republican Party of Wisconsin Fall Fest at the Walworth County Fairgrounds in Elkhorn, Wisconsin. (Photo by AFP)

Top US Republican Paul Ryan has said he would no longer defend his party’s presidential nominee Donald Trump, dealing a withering blow to the business mogul.

Ryan, the speaker of the House of Representatives, would instead use the next 29 days before the presidential election focusing on preserving his party’s hold on Congress, according to five lawmakers, who spoke on condition of anonymity.

In a morning conference call on Monday, Ryan told Republican lawmakers that he would never again campaign alongside Trump, said the lawmakers who participated in the call.

He said he would do his best to prevent Democratic presidential nominee Hillary Clinton from entering the White House, even though he effectively conceded defeat for his party in the presidential race, according to two of the lawmakers.

Trump speaks at a rally at Ambridge Area Senior High School on October 10, 2016 in Ambridge, Pennsylvania. (Photo by AFP)

Trump, who is under harsh criticism over his controversial conversations about women, acted indifferently in response to Ryan’s decision, giving him a belittling message on Twitter.

“Paul Ryan should spend more time on balancing the budget, jobs and illegal immigration and not waste his time on fighting Republican nominee,” Trump wrote in his Twitter post.

Representative Scott Rigell of Virginia, a Republican who has long been opposing Trump, supported Ryan’s stance, saying more humiliating disclosures about Trump were likely to come before the voting day.

“There’s a consensus, even among supporters, that the likelihood of something else breaking in a very embarrassing and negative fashion, is certainly better than 50-50,” said Rigell, who joined the call on Monday. “The conference, members, et cetera, are bracing themselves for another salvo of this.”

A recording, released on Friday by The Washington Post, shows Trump engaged in a lewd conversation about groping and seducing women.